A voltage-controlled oscillator (VCO) is a circuit used in many clocking components and systems such as phase locked loops (PLLs) and local oscillators (LOs). The objective of a VCO is to output a periodic signal (e.g., a clock signal) whose frequency is responsive to a control voltage. This control voltage is typically an analog signal and may be finely granular; therefore, it may also be known as the fine tuning control voltage.
The range of possible output signal frequencies is known as the VCO's frequency tuning range (FTR). A given circuit design will typically implement a VCO having the FTR chosen for the particular application of the given circuit. Accordingly, an industry trend towards wide frequency bandwidth transceivers creates a need for VCOs having a wide FTR.
Increasing a gain of the VCO is the simplest way to increase the FTR of the VCO. The gain of the VCO defines a change in output frequency versus a change in input voltage; therefore, greater VCO gain results in greater FTR.
However, a greater VCO gain also tends to amplify noise from various noise sources, which results in greater phase noise at the output of the VCO. From a noise-reduction perspective, it is generally desirable to reduce the gain of the VCO, which inevitably narrows the FTR.
In order to expand the FTR of a low-gain, low-noise VCO, the VCO may be configured to operate at a plurality of discrete frequency modes. Selecting one of these discrete operational modes (typically, by switching one or more discrete capacitors) may also be known as coarse tuning control.
Contemporary low-noise wide-FTR VCOs thus have both low-gain fine tuning control and discrete coarse tuning control. Typically, the VCO is configured with a plurality of capacitors selectively coupled to an inductor-capacitor (LC) tank via switches. The additional capacitances of the selectively coupled capacitors can greatly increase the overall FTR of the VCO without noticeably worsening the phase noise of the low-gain VCO in each discrete frequency mode.
However, these discrete operational modes of the VCO create one or more discontinuities in the FTR of the VCO, which prevents smoothly fine tuning the VCO output frequency between the discrete coarse tuning settings. In practice, the VCO should first be coarse tuned before the VCO is fine tuned. Initial coarse-tuning of the VCO may be known as start-up calibration. Correct start-up calibration is important in order to avoid undesirable re-calibration procedures.
For example, a system operating the VCO at a given coarse tuning setting may determine that the VCO needs to be tuned beyond the available range of the fine tuning control; in this case, the VCO will need to be coarse tuned, causing a jump in the VCO output frequency. This undesirable situation may arise, for example, when the VCO is not appropriately calibrated at start-up. The VCO may be calibrated to an initial temperature (or other operating condition) and after some time, a temperature drift (or change in operating condition) will cause the VCO to change its output frequency such that the frequency shift cannot be compensated for by the fine tuning control.
Therefore, while coarse tuning control allows a low-gain low-noise VCO to operate with a wider FTR, the VCO may be calibrated appropriately to minimize or eliminate undesirable instances of adjusting the coarse tuning control during VCO operation. The inventors have determined that improvements in calibration of VCOs and their control are desirable.